In a known hot-melt adhesive system, a stick of hot-melt adhesive material is cylindrical and has a diameter of substantially one-half inch. While it is not clear why this particular dimension was chosen originally, there are several factors which have dictated the use of a cylindrical stick having a diameter of approximately this dimension.
The cylindrical stick is manufactured through extrusion, and the circular cross section is maintained by twisting the stick as it comes from the extruder and passes to a cooling bath.
The one-half inch diameter allows a fairly efficient manufacturing process, and more sophisticated process controls are required to maintain the circular cross section as the diameter increases. In addition, if the diameter were larger than one-half inch, the tendency of the extruded stick to sag would increase, thus requiring additional steps in the manufacturing process. Moreover, a one-half inch stick cools on the outside fairly quickly to prevent sagging of the stick even while the inside is still soft. If the diameter were increased beyond one-half inch, the cooling of the outside surface would be inadequate to prevent sagging of the stick during the normal extrusion process. Further, if the diameter were increased, the glue stick would likely not cool (unless the cooling time were increased) before cutting or packaging which would result in further manufacturing and shipping problems.
The cylindrical glue stick shape is also critical to the design of a traditional glue gun. In the traditional glue gun, the glue stick is pushed into a heating chamber where it is melted. The unmelted portion of the glue stick thus acts as a seal for preventing the melted glue from flowing out of the heating chamber. Also, the hard glue stick is a piston which provides pressure to force the melted glue out of the nozzle. Thus, a good seal must be provided between the glue stick and the heating chamber. Glue which leaks out of the heating chamber cools in the mechanism of the gun and jams the mechanism of the glue gun.
The diameter and shape of the glue stick also affect the heating efficiency. In the heating chamber the glue stick is heated from the outside surface, and a round stick does not transfer heat efficiently because the surface area of the stick is small compared to its volume. Adhesive material is generally a thermal insulator, and one problem faced in the design of the heating chamber is that the center of the glue stick may still be hard when the outer portion is melted. One traditional solution to this problem is to provide fins in the heating chamber for penetrating the chamber and carrying heat to the center of the glue stick.
The shape and diameter of the glue stick also affects the choice of glue material. If the material is too soft, the glue stick will deform to such an extent that it prevents application of adequate pressure in the pressure chamber. If the glue stick is too hard, it will not deform enough to form the necessary seal with the heating chamber. If the diameter of a cylindrical glue stick is substantially less than one-half inch, the choice of material becomes critical because it must be formed of harder materials to have the necessary rigidity.
The shape of the glue stick also has an impact on the design of the glue gun. A stick is typically advanced into the heating chamber by a mechanism which grips the stick. If the adhesive material is hard, the gripping mechanism may not be able to engage the stick properly, thus preventing proper advancement of the stick into the heating chamber. If the material is too soft, the gripper may become embedded in the glue and not released properly at the end of the stroke, also preventing proper advancement of the stick. One objective is to distribute the gripping force over the stick, but this has not been practical with cylindrical sticks.